There are enhanced opportunities for applying an optical signal having merits such as a high speed, less electromagnetic noise and low loss to the signal transmission in LSI. Under such circumstances, various optical devices have been devised with a view to being integrated in LSI. One such device raising great expectations is a microring optical resonator which selectively drops (splits) a light of a single wavelength or lights of a plurality of wavelengths to light guide branch lines from a light guide main line or selectively adds a light of a single wavelength or lights of a plurality of wavelengths to the light guide main line from the light guide branch lines (see, for example, JP2006-504145). This is because a microring optical resonator can be integrated in LSI as an optical signal multiplexer/demultiplexer and has advantages such as a small size and a high speed in wavelength multiplexing optical transmission which enables high-volume signal transmission.
However, the diameter of even a small ring of a microring optical resonator as described in JP2006-504145 is, for example, 10 micrometers (μm), which is much greater than the circuit size (e.g., 1 μm) in modern LSI. Therefore, it may be difficult to densely integrate a plurality of microring optical resonators in LSI.